Every time a bell rings, an angel gets her wings.
Each Angelwing book tells the story of how a little angel earns her wings by being a helper to a child with a problem. The children's problems are simple and surmountable - ranging from a failure of courage to being clingy to being unkind to animals.
Each angel is especially suited to the task at hand, and is guided by a particular Archangel. Thus, the Little Angel of Courage (who herself could be braver) helps Paul summon up the courage to take ballet lessons; the Little Angel of Freedom helps Simone figure out that being in a cage is harmful to frogs and grasshoppers, the Little Angel of Acceptance helps TJ overcome his stutter.